1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus represented by a copier or a printer and particularly to an image forming apparatus for performing the primary transfer of a toner image formed using toners of a plurality of different colors from an image bearing member to the outer surface of an intermediate transfer member and the secondary transfer of the toner image from the outer surface of the intermediate transfer member to a sheet.
2. Description of the Related Art
A method for transferring a toner image by directly bringing a sheet into contact with a photoconductive drum or a method for performing the primary transfer of a toner image to an intermediate transfer member constructed by another drum or a belt and then the secondary transfer of the toner image transferred to the intermediate transfer member to a sheet are known as a method for transferring a toner image formed on the outer circumferential surface of a photoconductive drum as an image bearing member to a sheet in an electrophotographic image forming apparatus. Out of these methods, the latter method using the intermediate transfer member is widely used in the case of full-color printing using a plurality of different colors. In such a case, a superimposed toner image is formed on the outer surface of the intermediate transfer member by successively transferring toner images of a plurality of colors to the intermediate transfer member as the primary transfer and finally transferring the thus formed superimposed toner image to a sheet as the secondary transfer.
According to such a transfer method, a minute amount of toner remains on the outer surface of the intermediate transfer member without being transferred after the secondary transfer of the toner image to the sheet in some cases. Since the residual toner on the outer surface of the intermediate transfer member hinders a new image forming operation, the residual toner needs to be cleaned. A method for pressing a rotary member such as a roller or a rotary brush against the outer circumferential surface of the intermediate transfer member to transfer the residual toner to the rotary member for collection, a method for scraping the residual toner off by bringing a blade into contact with the outer surface of the intermediate transfer member and a cleaning method as a combination of the above two methods are widely known as cleaning methods used for such a purpose.
In the case of using a rotary member such as a roller or a rotary brush in the cleaning method for cleaning the residual toner on the outer surface of the intermediate transfer member, a cleaning bias having a polarity opposite to that of electric charges of the toner is normally applied to the rotary member during the cleaning. As a result, the residual toner on the outer surface of the intermediate transfer member can be transferred to the rotary member by an electrostatic force, whereby the residual toner can be easily cleaned and collected.
However, there are cases where the residual toner on the intermediate transfer member is charged to have an opposite polarity due to the injection of transfer charges upon the transfer of the toner image to the sheet through the application of a secondary transfer bias, separating discharge that occurs upon separating the sheet from the intermediate transfer member, and the like. If the residual toner on the intermediate transfer member is charged to have an opposite polarity, it has the same polarity as the cleaning bias applied to the rotary member such as a roller or a rotary bush at the time of cleaning the residual toner, making it difficult to clean and collect the residual toner. If the toner, which cannot be cleaned, remains on the outer surface of the intermediate transfer member, a next image forming process is adversely affected and there occurs an image error of attaching the toner to parts of the sheet that are supposed to be pure while without having no image.
In view of the above, a cleaning method has been proposed which has a good cleaning performance by devising a method for applying a cleaning bias to residual toner, which can be neither cleaned nor collected due to a polarity change as above, on the outer surface of the intermediate transfer member. One example of such a method is disclosed in Japanese Unexamined Patent Publication No. 2005-17424 (pages 4 to 5, FIG. 1).
According to a method for cleaning an intermediate transfer member of an image forming apparatus disclosed in this publication, a charging roller held in contact with the intermediate transfer member is provided without providing residual toner collecting means such as a cleaning device for the intermediate transfer member, and the toner residual on the outer surface of the intermediate transfer member is entirely charged at the same polarity opposite to a normal charged polarity, so that the residual toner can be collected by the cleaning device for an image bearing member provided in an image forming units. However, it is normally difficult to uniformly charge the entire toner residual on the outer surface of the intermediate transfer member as in this method and there is a high possibility that some of the toner cannot be charged to have the opposite polarity. Thus, the toner, which cannot be completely cleaned, remains on the outer surface of the intermediate transfer member, leading to a likelihood that a next image forming process is adversely affected and an image error occurs as described above.